(* Copyright (c) 1993-1997, Silicon Graphics, Inc.
 * ALL RIGHTS RESERVED 
 * Permission to use, copy, modify, and distribute this software for 
 * any purpose and without fee is hereby granted, provided that the above
 * copyright notice appear in all copies and that both the copyright notice
 * and this permission notice appear in supporting documentation, and that 
 * the name of Silicon Graphics, Inc. not be used in advertising
 * or publicity pertaining to distribution of the software without specific,
 * written prior permission. 
 *
 * THE MATERIAL EMBODIED ON THIS SOFTWARE IS PROVIDED TO YOU "AS-IS"
 * AND WITHOUT WARRANTY OF ANY KIND, EXPRESS, IMPLIED OR OTHERWISE,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTY OF MERCHANTABILITY OR
 * FITNESS FOR A PARTICULAR PURPOSE.  IN NO EVENT SHALL SILICON
 * GRAPHICS, INC.  BE LIABLE TO YOU OR ANYONE ELSE FOR ANY DIRECT,
 * SPECIAL, INCIDENTAL, INDIRECT OR CONSEQUENTIAL DAMAGES OF ANY
 * KIND, OR ANY DAMAGES WHATSOEVER, INCLUDING WITHOUT LIMITATION,
 * LOSS OF PROFIT, LOSS OF USE, SAVINGS OR REVENUE, OR THE CLAIMS OF
 * THIRD PARTIES, WHETHER OR NOT SILICON GRAPHICS, INC.  HAS BEEN
 * ADVISED OF THE POSSIBILITY OF SUCH LOSS, HOWEVER CAUSED AND ON
 * ANY THEORY OF LIABILITY, ARISING OUT OF OR IN CONNECTION WITH THE
 * POSSESSION, USE OR PERFORMANCE OF THIS SOFTWARE.
 * 
 * US Government Users Restricted Rights 
 * Use, duplication, or disclosure by the Government is subject to
 * restrictions set forth in FAR 52.227.19(c)(2) or subparagraph
 * (c)(1)(ii) of the Rights in Technical Data and Computer Software
 * clause at DFARS 252.227-7013 and/or in similar or successor
 * clauses in the FAR or the DOD or NASA FAR Supplement.
 * Unpublished-- rights reserved under the copyright laws of the
 * United States.  Contractor/manufacturer is Silicon Graphics,
 * Inc., 2011 N.  Shoreline Blvd., Mountain View, CA 94039-7311.
 *
 * OpenGL(R) is a registered trademark of Silicon Graphics, Inc.
 *)

(*  alpha.ml
 *  This program draws several overlapping filled polygons
 *  to demonstrate the effect order has on alpha blending results.
 *  Use the 't' key to toggle the order of drawing polygons.
 *)

(* Ported from C to OCaml by Florent Monnier *)

open GL
open Glu
open Glut

let leftFirst = ref true


(*  Initialize alpha blending function. *)
let init() =
  glEnable GL_BLEND;
  glBlendFunc Sfactor.GL_SRC_ALPHA  Dfactor.GL_ONE_MINUS_SRC_ALPHA;
  glShadeModel GL_FLAT;
  glClearColor 0.0 0.0 0.0 0.0;
;;

let drawLeftTriangle() =
  (* draw yellow triangle on LHS of screen *)

  glBegin GL_TRIANGLES;
    glColor4 1.0 1.0 0.0 0.75;
    glVertex3 0.1 0.9 0.0;
    glVertex3 0.1 0.1 0.0;
    glVertex3 0.7 0.5 0.0;
  glEnd();
;;

let drawRightTriangle() =
  (* draw cyan triangle on RHS of screen *)

  glBegin GL_TRIANGLES;
    glColor4 0.0 1.0 1.0 0.75;
    glVertex3 0.9 0.9 0.0;
    glVertex3 0.3 0.5 0.0;
    glVertex3 0.9 0.1 0.0;
  glEnd();
;;


let display() =
  glClear [GL_COLOR_BUFFER_BIT];

  if !leftFirst then begin
    drawLeftTriangle();
    drawRightTriangle();
  end else begin
    drawRightTriangle();
    drawLeftTriangle();
  end;

  glFlush();
;;


let reshape ~width:w ~height:h =
  glViewport 0 0 w h;
  glMatrixMode GL_PROJECTION;
  glLoadIdentity();
  if w <= h 
  then gluOrtho2D 0.0 1.0 0.0 (1.0 *. float h /. float w)
  else gluOrtho2D 0.0 (1.0 *. float w /. float h) 0.0 1.0;
;;


let keyboard ~key ~x ~y =
  match key with
  | 't'
  | 'T' ->
      leftFirst := not !leftFirst;
      glutPostRedisplay();	
  | '\027' ->  (*  Escape key  *)
      exit(0);
  | _ -> ()
;;


(*  Main Loop
 *  Open window with initial window size, title bar, 
 *  RGBA display mode, and handle input events.
 *)
let () =
  let _ = glutInit Sys.argv in
  glutInitDisplayMode [GLUT_SINGLE; GLUT_RGB];
  glutInitWindowSize 200 200;
  let _ = glutCreateWindow Sys.argv.(0) in
  init();
  glutReshapeFunc ~reshape;
  glutKeyboardFunc ~keyboard;
  glutDisplayFunc ~display;
  glutMainLoop();
;;

